The invention concerns an extended-nip press for dewatering of a paper board or board web, comprising a counter-member, most appropriately a press roll, which forms an extended press zone together with a press-glide shoe and a glide belt running around the shoe, through which said zone at least one dewatering fabric and the web from which water is removed and which is supported by said fabric and passed, and which glide belt is passed over a leading and tensioning member.
In the prior art, extended-nip presses are known wherein the press zone is formed between a revolving belt mantle provided with a stationary core and a counter-roll. As is well known, in the stationary roll core hydraulically or hydrodynamically loaded pressure shoes are used, by whose means, by the intermediate of the revolving mantle, a compression pressure is applied to the web towards the counter-roll. The counter-roll may be either an ordinary smooth-faced or hollow-faced press roll, a variable-crown roll, or a belt mantle provided with a hydraulic glide shoe. As an example of the prior-art extended-nip press described above, reference is made to the GB Patent Application No. 2,057,027 as well as to the published Patent Applicant WO 82/02567.
In the patent applications mentioned above, no satisfactory solution has been suggested for the construction of the ends of the roll provided with a stationary core and with a revolving belt mantle. The object of said sealed ends is to prevent access of oil out of the interior of the roll mantle, e.g., to spoil the paper web. Lubrication fluid is needed in the lubrication between the inside face of the revolving mantle and the glide shoe or shoes that guide the mantle in considerable amounts. In this respect, reference is made to the U.S. Pat. No. 3,804,707, wherein an extended-nip press is described in which said roll ends are stationary and provided with resilient seal rings that rub against the inner face of the flange in the area of the ends of said roll mantle. Since the roll mantle has to alter its shape from the circular shape during each round of its rotation especially in the nip zone, this causes a considerable fatiguing and wearing load on said seal rings.
With respect to the prior art related to the present invention, reference is made further to the FI Patent Applications 821503 and 850213, to the FI Patent 66,932, to the DE Published Pat. Appl. 3,239,954, and to the U.S. Pat. No. 4,584,059.
One drawback, e.g., in the extended-nip presses known from the publications cited above is the lateral wandering of the belt mantle, because the prior-art devices have lacked efficient guide means for the belt mantle to keep the belt stably in its place laterally.
The press forces employed in extended-nip presses are of an order of 10.sup.7 N, in which case a fully carrying lubricant film must be provided between the glide shoe and the belt mantle. In such a case, water is not adequate as a lubricant, but it is necessary to use different lubrication oils and hydrodynamic or hydrostatic lubrication chambers, the pressure level employed in said chambers being of an order of 4-8 MPa. The thickness of the belt mantles used is about 3-6 mm. This is why they can be guided exclusively by drawing, which also causes its problems in the guiding of the belts.
Long belt mantles are preferable to short circular hose mantles in the respect that their service life is considerably longer and the standstills resulting from replacement of belt mantle are less frequent. Thus, one object of the present invention is to make the intervals between said standstills longer.
An object of the present invention is to combine the good properties of a belt-mantle construction closed at its ends and of an open belt.
An object of the invention is to provide such an extended-nip press belonging to the species concerned wherein the opening gap of the nip can be made sufficiently large, as a rule about 30-50 mm, so that the glide belt and the press felts can be replaced as quickly as possible.
The main object of the present invention is to provide an extended-nip press belonging to the species defined at the beginning which is also suitable for thin paper qualities and for high machine speeds.
The employment of the prior-art extended-nip presses of the sort concerned at high running speeds and with thin qualities has been prevented completely or at least restricted, e.g., by the following circumstances. In a construction with an open long glide belt, oil leakages and oil mist are increased as the speed becomes higher. Instead, the glide belt has a relatively good service life (up to 6 months), because its bend-loading situation is favorable. Leakages of oil in a belt-mantle construction closed at the ends are small, even though this construction is not fully sealed either. Relatively short service life of the belt mantle can be a problem even at lower speeds, because the bend-loading situation of the belt mantle is difficult. The mantle has to bend to a "concave" shape in the area subjected to nip load as well as to a "convex" shape in the end area constructing an extension of the nip. Moreover, the circumferential dimension of the belt mantle is small and the frequency of load alternation becomes high.